Yes, it ruins nationalism
No, you can still have nationalism
It doesnt ruin it but it does weaken its influence slightly, which I consider a good thing.
I still love the USA yet I learned japanese.
I know two other languages (English and French) other than my native dialect and the standard dialect, yet I'm still nationalistic. :)
I can speak English, Walloon, Creole, French, Spanish, Italian, German, and a bit of Persian. I'm still an American. Learning French was more toward my ethnicity than my nationality, but the rest was fun.